To ionize molecules or atoms for the analysis in a mass spectrometer or a mobility spectrometer different ionization techniques are employed. Many of these techniques provide ions within a cloud from which only those can be investigated that enter the mobility spectrometer or the mass spectrometer through some narrow orifice. In some cases a double ion analysis is required and the ions must be introduced through a small orifice into a mobility spectrometer at approximately atmospheric gas pressure and then from the exit of this mobility spectrometer through another small orifice into an evacuated mass spectrometer. To guide ions through one or through several small orifices is always difficult to achieve so that commonly a large percentage of the formed ions will impinge on the sides of said orifice and be lost for the analysis
When ions are formed in gas at a pressure that is higher than the pressure in the mobility spectrometer or mass spectrometer the effect of the gas flow into this ion analyzer must be taken into account also. Thus, orifices are often formed as a skimmer that has sharp edges mainly because this reduces the effects of gas turbulence.
Representative atmospheric pressure ionization is achieved in an “atmospheric pressure electrospray ionization” (ESI) or an “atmospheric pressure chemical ionization” (APCI). In the ESI method a voltage of several kV is applied to the nozzle of a capillary to which a liquid sample is applied. At this nozzle small charged droplets are formed from which the solvent evaporates quickly leaving portions of the droplet charge on the initially dissolved molecules. In the APCI method a needle is aligned to this nozzle that initiates a corona discharge which ionizes atoms or molecules of the carrier gas which after a very short time transfer their charge to molecules of interest. In both methods often the nozzle and/or the carrier gas is heated so as to enhance the evaporation rate of the droplets since still intact droplets would be detrimental to the functioning of the mobility spectrometer or the mass spectrometer.
In case the ions are introduced into an evacuated mass spectrometer the gas flow should be reduced so much that the pumping capacity suffices. This can be achieved for instance by a straight or curved capillary (see Patent Document 1) which can also be heated in order to assist the evaporation of residual droplets. However, in most cases only a portion of the formed ions enter the capillary and even of these many will interact with the walls of the capillary and thus be lost. Some improvement of this method is obtained when this capillary is replaced by a skimmer or sampling cone (see Patent Document 2). In both methods, however, only a portion of the formed ions can be utilized.
In order to increase the ion transmission into the evacuated mass spectrometer also configurations have been used (Patent Documents 3, 4 and 5) in which not a single but several apertures were used.    [Patent Citation 1] Japanese Unexamined Patent Application Publication No. H7-68517    [Patent Citation 2] Japanese Unexamined Patent Application Publication No. H8-304342    [Patent Citation 3] U.S. Pat. No. 6,818,889    [Patent Citation 4] U.S. Pat. No. 6,949,740    [Patent Citation 5] U.S. Patent Application Publication No. 2004/0245458